Traditionally, there have been technological obstacles to providing efficient IP over satellite links. Tachyon engineering has addressed and overcome these issues. It is known that TCP/IP performs poorly over high-latency or noisy channels. Geostationary satellite links are inherently high-latency, and can be noisy. Historically, inexpensive satellite terminals have been used primarily for telephony-type applications where noise is less of an issue. Bit Error Rates (BER) of 10 -7 can be acceptable in telephone environments, but this level of performance renders TCP/IP almost unusable.

The TCP/IP shortcomings in the typical satellite environment - degradations due to slow-start, window size, and acknowledgment frequency - are well known. There have been attempts to deliver IP over satellite, but the satellite technologies have focused on connection-oriented transmission protocols that are really suited for voice traffic rather than IP, and unnecessarily squander expensive capacity.

The Tachyon development team has overcome these traditional IP-over-satellite technology difficulties. The Tachyon solutions are presented in the following sections.

ISSUE-Latency TCP/IP in conjunction with a high-latency satellite channel has some obvious and significant problems. The round-trip latency of the satellite link is 0.5 seconds. TCP/IP has an inherent mechanism called slow-start to discover the channel throughput capacity upon initial connection setup. Slow-start sends a packet across the channel and waits for a response. If a response is received, the next packet is sent a bit faster. This procedure is repeated until the speed of the link is discovered. With the half-second delay between responses, throughput is significantly slowed. Clearly, if slow-start can be bypassed, a significant drag on TCP/IP performance can be removed.

SOLUTION-Enhanced Satellite Link Protocol To overcome slow-start, Tachyon implemented an enhanced communications protocol over the satellite link.
In the Tachyon network all TCP connections are terminated and re-originated at both ends of the satellite link. Over the satellite link, a new protocol is used that is both transparent to TCP/IP and optimized for the satellite environment.
This satellite link protocol does not use slow-start. Since Tachyon controls the link and schedules capacity, the throughput for a TCP connection is known prior to transmission and no discovery process is required. The data is sent at a predetermined rate over the link without any round-trip delays.

ISSUE-Window Size TCP/IP has a built-in windowing mechanism that is intended to achieve the highest possible throughput, while balancing the risk of retransmitting dropped packets. It works by allowing a transmitter to send a number of packets before having to wait for an acknowledgment from the receiver. Typically, the window size is set to accommodate low-latency (terrestrial) channels of uneven noise performance (e.g., POTS lines), yet not hold transmission up for processing delays at either end of the link. So the windows tend to be short with respect to the 0.5-second delay of a satellite link, but minimize the amount of data that would need to be retransmitted when packets are dropped. Short windows can slow down a satellite connection dramatically. Opening up the constricted flow of packets requires adjusting (tuning) window size for the known latency and expected noise performance of the link.

SOLUTION-Reduce BER, Optimize Window Size The Tachyon network has reduced Bit Error Rates to rates compatible with good TCP/IP performance. The Tachyon satellite link protocol includes a mechanism to optimize window size to the satellite link. The characteristics of BER and latency on the satellite link are known, so an appropriate window size can be selected. Figure 5 illustrates typical performance over the Tachyon network. Under heavy rain fade conditions, the Tachyon network will perform at ~10 -8 BER. Under clear-sky conditions, the Tachyon network will perform at 10 -10 BER or better. At these performance levels, very large window sizes are possible, which effectively removes throughput degradation due to window size. The low BER also allows a reduced frequency of acknowledgments, for increased network efficiency.

ISSUE-Acknowledgment Traffic Latency is exacerbated in TCP/IP transmissions by the fact that the protocol requires extensive handshaking across the link. Most packets sent across a TCP/IP link have a corresponding acknowledgment packet that is sent back to confirm receipt. This acknowledgment system does ensure reliable end-to-end communication under uncertain and congested network conditions. However, the acknowledgment trafficking becomes a problem for short transmissions over high-latency channels. Each packet exchange takes at least 0.5 seconds to complete. While this may be a smaller factor in broadcast-only applications, it can significantly limit throughput for typical interactive IP applications such as Internet, Intranet, and Extranet. Improving TCP/IP performance over a high-latency link means reducing the frequency of acknowledgments.

SOLUTION-Enhanced TCP/IP Given the excellent BER performance of the Tachyon network, the frequency of acknowledgment traffic can be greatly reduced as well without sacrificing reliability. The low Bit Error Rate of the Tachyon network and resulting reduced acknowledgment frequency contribute to reducing the drag on TCP/IP performance due to latency. Figure 6 summarizes the results of Tachyon’s TCP/IP performance enhancements. As shown, raw TCP/IP, even at good BER levels, only performs at ~20% throughput over a satellite link. With selective acknowledgment, a common TCP stack enhancement, throughput only increases to ~25%. The Tachyon network, by contrast, operates at ~78% of link capacity. Also note that as BER performance degrades with rain attenuation, the Tachyon network degrades at a much slower rate than either TCP/IP or TCP/IP with selective acknowledgment.

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